1. Technical Field
The present invention relates to an optical film and a polarizing plate contributing toward optical compensation in VA-mode liquid-crystal display devices, and to a liquid-crystal display device comprising it.
2. Background Art
Applications of liquid-crystal display devices are expanding year by year as power-saving and space-saving image display devices. Heretofore, one serious defect of liquid-crystal display devices is that the viewing angle dependence of image is large. Recently, however, various types of wide viewing angle-mode liquid-crystal display devices where the liquid-crystal molecules in the liquid-crystal cell align in the different manner from the previous one have been put into practical use, and in that situation, the demand for liquid-crystal display devices is rapidly expanding even in the market such as the TV market which require wide viewing angle characteristics.
In general, a liquid-crystal display device comprises a liquid-crystal cell, an optical compensation film and a polarizing element. The optical compensation film acts to solve the problem of color shift and to widen viewing angles, for which used is a stretched birefringent film or a film produced by coating a transparent film with a liquid crystal. For example, Japanese Patent No. 2587398 discloses a technique of applying an optical compensation film produced by applying a discotic liquid crystal onto a triacetyl cellulose film and aligning and fixing their molecules thereon, to a TN-mode liquid-crystal cell to thereby widen the viewing angle of the device. However, for liquid-crystal display devices for TV applications that are expected to be watched at various angles on large panels, the demand for reducing the viewing angle dependence y is serious, and even the above-mentioned technique could not satisfy the requirement. Accordingly, other modes of liquid-crystal display devices differing from TN-mode devices, for example, IPS (in-plane switching)-mode, OCB (optically compensatory bend)-mode or VA (vertically aligned)-mode devices are now under investigation.
In particular, VA-mode devices have a high contrast and their producibility is relatively good, and therefore VA-mode devices are specifically noted for liquid-crystal display devices for TV applications. However, VA-mode devices have a problem in that, though they can provide almost complete black states in the direction along the normal line relative to the panel plane, light leakage may occur in watching the panel in oblique directions, and therefore the viewing angle is narrow.
To solve the problem, a technique is reported, which comprises suitably designing the relationship between the Re value and the Rth value of the polarizing plate-protective film on the side of the liquid-crystal cell and Δnd of the liquid-crystal cell to fall within a controlled range, and suitably designing the coloration level in an orthogonal disposition of polarizing elements as well as the color temperature of the backlight in the liquid-crystal display devices to fall within a controlled range, thereby widening the viewing angle and reducing the color shift in the black state (for example, see JP-A 2007-140497). In addition, also reported is another technique comprising disposing a retardation film between a liquid-crystal cell and a polarizing film, controlling the wavelength dispersion characteristics of the retardation film, and using a plurality of specific retardation films as combined, thereby providing almost colorless black states with little light leakage throughout the entire visible light region (for example, see Japanese Patent No. 3648240). In addition, WO2004/068226 proposes a technique of using a retardation film having reversed wavelength dispersion characteristics of Re and having regular wavelength dispersion characteristics of Rth, but not using plural retardation films as combined, thereby providing almost colorless black states; however, any concrete means for realizing the technique is not provided.